The present invention relates to a metallic cord for reinforcing rubber articles such as pneumatic tires in which penetration of rubber into the cord is improved.
Many rubber articles such as pneumatic tires and the like are generally reinforced with steel cords.
As shown in FIG. 15, a conventional steel cord is formed by compactly twisting straight steel filaments together. This type of steel cords however have a defect such that closed space not filled with rubber is formed among the filaments and steel filaments are liable to rust and adhesion between the cords and rubber becomes insufficient. As a result, the cord strength greatly decreases and the cord is broken finally.
Recently, in order to improve the penetration of rubber into cords to solve the above-mentioned problems, there have been proposed a steel cord (b) in which straight filaments (f) are loosely twisted as shown in FIG. 16, and a steel cord (c) in which spiral filaments (f1) and straight filaments (f2) are twisted as shown in FIG.
Such cords however, necessitate a relatively large cord diameter in order to secure stable rubber penetration. Further, such cords tend to show a large initial elongation. As a result, it is difficult to use the cords in order to reinforce a carcass and beads of pneumatic tires.
It is therefore, an object of the present invention to provide a steel cord in which the rubber penetration is improved without increasing the cord diameter and initial elongation.
According to the present invention, a steel cord is formed by compactly twisting steel filaments together, the steel filaments including a plurality of zigzag filaments and optionally a nonzigzag filament, the zigzag filament being zigzaged two-dimensionally and made up of straight segments each extending between the zigzag peak points, the nonzigzag filament being straight. In case the cord is composed of only the zigzag filaments, the zigzag filaments include at least two kinds of zigzag filaments having different zigzag pitch lengths. And at least two kinds of zigzag filaments are first loosely twisted together into a bunch, and then a plurality of bunches are twisted together into the cord. In case the cord is composed of both the zigzag filaments and nonzigzag filaments, some of the zigzag and nonzigzag filaments are first loosely twisted together into a bunch, and then a plurality of bunches are twisted together into the cord. In any case, the zigzag pitch length and zigzag wave height of each zigzag filament are substantially constant along the length thereof.
A tire comprises a reinforcing layer such as a tread reinforcing belt layer, carcass, bead reinforcing layer and the like made of the above-mentioned steel cords.